Injectable hydrogels are emerging as promising materials for biomedical applications like drug delivery because of their biocompatibility, ease of administration and minimal invasion due to their high resemblance with natural extracellular matrices. Bioactive molecules like drugs, proteins, DNA and antibodies can be easily mixed with precursor solutions and loaded at target site via an in-situ gelation right after the injection. The release of these bioactive molecules can be performed in a sustainable or burst way on demand in response to external stimuli such as change in temperature or pH, introduction of redox or biomolecules, and exposure to light or electric field. Encapsulated in hydrogel matrix, the loaded molecules can be maximally protected from unnecessary enzymatic degradation or hydrolyzation to retain their bioactivity, before triggered for releasing to target cells or tissues by external stimuli to fulfil their therapeutical potential. However, proteins or microorganisms could easily adhere to implanted hydrogels and form biofouling films, not only blocking the circulation of loaded biomolecules but also triggering an immune response or inflammation.
A common means to address this challenging issue is to confer the developed hydrogels antifouling or antimicrobial properties to minimize accumulation of biofouling films on their surfaces. Nonetheless, the outcome of this approach is quite limited because implanted hydrogels after injection suffer from constant external mechanical force, which could lead to certain deformation or damage of the hydrogels. Once disruption takes place in vivo, body fluids will intrude and simultaneously introduce nutrients and microorganisms to build up detrimental biofoulings, consequently shortening the lifespan of the hydrogel materials used and inducing further inflammatory responses. In this circumstance, a hydrogel possessing autonomous healing capability after inflicted damage will be of great significance to extend its application and lifespan, because the integrity of the broken hydrogel fragments after injection could be recovered at the target site under physiological conditions, preventing a burst release of the loaded biomolecules and enhancing delivery efficiency. The healable networks are usually constructed through interactions such as dynamic covalent bonding, noncovalent linkages, host-guest interactions and hydrogen bonding. Recently marine mussel has inspired various applications in diverse fields, among which the preparation of self-healing hydrogels inspired by the self-repair of mussel threads is of great significance. Marine mussels secrete foot proteins which after a curing process can form byssus consisting of proteinaceous thread and adhesive plaque to adhere to various substrates underwater. The self-repair of mussel byssal threads is mainly attributed to the reversible metal-catechol coordination between metals like Fe3+ and catechol groups from an amino acid called 3,4-Dihydroxyphenyl-L-alanine (DOPA), and other interactions like cation-π interaction can also play a role. Very recently, self-repair was also demonstrated in metal-free water of synthetic polyacrylate and polymethacrylate surface-functionalized with catechols through catechol-mediated interfacial hydrogen bonds.